OTFT transistors are electronic devices in which the semiconductor layer is made of organic material.
The research in this field has led to the development of OTFT transistors in which, in addition to the semiconductor layer, the dielectric layer and the source, drain, and gate contacts are also made of organic materials, respectively.
OTFT transistors are a valuable alternative to traditional inorganic thin film transistors.
OTFT transistors, similarly to traditional Metal Oxide Semiconductor Field Effect Transistor transistors (MOSFET) have a semiconductor layer (in which the conductive channel is formed) that is deposited on a dielectric layer. Most of OTFT transistors presently known in the literature are so-called p-channel devices.
OTFT transistors are usually classified according to two major categories based on the position of the source and drain contacts relative to the semiconductor layer: top contact configuration, and bottom contact configuration.
In the top contact configuration, the source and drain contacts are placed in contact with the upper surface of the semiconductor layer and extend outside the latter.
In the bottom contact configuration, the source and drain contacts are placed in contact with the lower surface of the semiconductor layer and extend within the latter. In both configurations, the gate contact is deposited on the substrate, as already traditionally occurs with devices of this type.
In the fabrication of OTFT transistors in the bottom contact configuration, the organic semiconductor layer has been noted to be of worse quality, and accordingly worse performance, than the top contact configuration. This implies that, at present, top-performing OTFT transistors are manufactured in a top contact configuration.
OTFT transistors are currently competitive in those applications that require the coverage of large areas, flexible structures, forming process at room temperature, low-cost manufacturing technology. In fact, the organic materials allow, for example, employing techniques for manufacturing large-area devices and are compatible with flexible substrates, i.e. made of a material other than conventional silicon or glass (rigid substrates), i.e. organic material (such as PEN film, polyethylene naphtalate) or even paper sheets.
The techniques used for the deposition of organic material mainly fall in two categories: solution casting and high-vacuum thermal deposition. Depositions techniques such as solution casting, dip-coating, spin-coating and printing belong to the first category.
An important aspect in the manufacture of OTFT transistors is the geometry definition in the semiconductor layer. In fact, the geometry definition in the organic material layer is quite complicated, as subsequent treatments on the organic material will cause a degradation of the properties thereof. For example, for several applications, a deposition technique by means of an auxiliary mask (known as the “shadow mask”) is either employed, or the organic material is mechanically removed from around the geometry to be defined. Both cases have drawbacks related to the fact that high area densities cannot be obtained, as well as other problems due to the use of masks, which require to be cleaned after each use, and in case of high-resolution type, result to be thin and easily breakable.
Alternatively to the photolithographic techniques (or however those requiring a mask), techniques have been recently developed, which combine the deposition and definition of the geometries of the semiconductor layer in an individual step (for example, the technique of solution casting by printing).
Furthermore, among the low-cost techniques, those belonging to the family of non-photolithographic techniques known as the soft-lithography are widely used. It comprises a number of techniques used for manufacturing, for example, high-quality microstructures and nanostructures using masks, molds and elastomeric stamps to define geometries in a determined layer and transfer them to the device substrate. The primary soft-lithographic techniques known in the literature are: Microcontact printing (pMC), Micromolding in capillaries (MIMIC), Replica molding (REM), Microtransfer molding (μTM) and Solvent-assisted micromolding (SAMIM).
However, the manufacturing techniques known as yet, though having appealing aspects, still do not allow OTFT transistors to be fabricated such as to conjugate the requirement of low manufacturing costs with a satisfactory electric performances of the resulting OTFT transistors.